There exists the need for a rapid, safe, accurate and non-intrusive inspection system for determining the presence of contraband in parcels, luggage, vehicles, and the like, both above and below the surface of the earth. As used specifically herein, contraband shall refer to hazardous materials such as explosives and narcotics, and shall include military explosives such as mines and ammunition. While it is theoretically possible to inspect every parcel or item passed across a border, through an air, rail or sea terminal, or through a post office, such inspection would be very costly, waste time and impede commerce; furthermore, smugglers frequently resort to the use of packages having hidden compartments, false bottoms and the like, which may be overlooked in all but the most scrupulous inspection. There is also a need for explosive detection techniques which may be adapted to scan quickly, the surface or subsurface of the earth for mines or buried explosives. It is preferred that such techniques be reliable, rapid and capable of conducting such inspection from a distance, as for example in a flyover inspection. Accordingly, there is a need for a rapid method for detecting the presence of contraband and it is preferred that the technique be non-invasive or non-visual, that is to say, that it be capable of inspecting the contents of a closed container without necessitating its opening. In many instances a container will not be "closed" in the sense of being sealed, but may be partially opened porous or permeable; however, as used herein, a closed container shall include all such containers not readily subject to visual inspection and shall include parcels, packages and envelopes as well as structural components of buildings and vehicles as well as subsurface or otherwise camouflaged objects.
A variety of techniques have been proposed for accomplishing such inspections. In some instances, thermal neutron activation analysis (TNAA) has been employed. In this technique, relatively low energy neutrons are employed to bombard a sample under investigation. The nuclei of component atoms thereof capture these neutrons and become radioactive. These newly formed radioactive isotopes then undergo gamma decay and emit photons in the process. By identifying the emitted radiation, the composition of the sample may be determined. TNAA techniques are not well suited for the high volume inspection of closed containers since the technique, of necessity, renders the sample being inspected radioactive, and this radioactivity may persist for a significant period of time after completion of the analysis, thereby presenting a potential health hazard. In many instances, the exact composition of the sample under investigation is not known, and consequently the duration of the induced radiation cannot be told beforehand. Furthermore, TNAA techniques are not particularly efficient for detecting nitrogen or carbon, major components of narcotics and explosives, because the capture cross section for these elements is quite small as compared to that of metals and other heavy elements.
In another technique, for example that disclosed in U.S. Pat. No. 5,076,993, an object under investigation is bombarded with fast neutrons having an energy in the range of 5-9 MeV. The neutrons induce the emission of gamma rays from the object and the energy of the emitted gamma rays is correlated with the particular nucleus.
U.S. Pat. Nos. 4,864,142 and 4,918,315, the disclosures of which are both incorporated herein by reference, disclose analytical techniques based upon the elastic scattering of neutrons. The techniques disclosed therein are quite attractive since they do not induce any residual radioactivity in the target elements; also, the elastic scattering cross section for neutrons is much larger than the absorption cross section, and this difference is greatest for the light elements where scattering cross sections are typically 100 to 1,000 times greater than absorption cross sections. Such large cross sections make possible the use of relatively low fluxes of neutrons for scattering analyses. Resonant elastic scattering techniques are of particular interest since they can be highly specific for particular elements. Each element has a unique elastic scattering spectrum characterized by the presence of resonance peaks therein. These resonance peaks represent particular neutron energies at which the elastic scattering cross section of a given element is large. Particular peaks may be correlated with the presence of specific elements.
As detailed in the aforementioned patents, resonance based analyses were heretofore carried out by impinging a monoenergetic neutron beam onto a sample, detecting the elastically scattered neutrons, varying the energy of the monoenergetic beam and making further measurements. A resonance peak will be manifested by a very high scattering cross section, compared to the scattering cross section at other energies. The energy could be varied in a continuous way so as to generate a spectrum; however, in a more practical apparatus, neutron energies of two or more discrete levels were employed to detect specific elements. For example, in order to detect a particular preselected element, a first neutron beam having an energy corresponding to a resonance scattering energy of that element was directed onto the object, and the intensity of elastically scattered neutrons measured. Then, a second beam of neutrons having an energy that will not be resonantly scattered from the preselected element was directed onto the object, and the intensity of scattered neutrons measured. By comparing the two scattering intensities, resonant scattering could be determined. In a practical sense, most objects being investigated will include a number of different elements therein; consequently, the first and second energy levels are chosen so that interference is minimized; i.e., the first energy level is selected so that only the preselected element has a significant resonant scattering thereat, and the second energy level is selected so that none of the elements present in the object has any significant resonant scattering thereat. As is further detailed in the referred-to-patents, the technique may be implemented to determine the presence of multiple elements in an object by selection of the appropriate resonance and reference neutron beams.
It is also possible to determine the relative ratios of preselected elements in an object by using this resonant scatter technique. Various contraband items such as explosives and narcotics have distinct compositions, and the presence of particular elements, such as nitrogen, may be correlated with these items. Furthermore, narcotics, and explosives have rather particular ratios of carbon, oxygen and nitrogen and these ratios may be used to distinguish these materials from one another as well as from ordinary items of commerce, which may also include the aforementioned elements in different proportions.
The monoenergetic neutrons are typically generated by impacting a target material (e.g. D, Li, etc.) with an ionized beam of another material, typically, D or H. By control of the accelerating voltage, the energy of the emitted neutrons may be selected. In many accelerator systems it is somewhat difficult to switch voltage from a first to a second, and back to a first level accurately and quickly. Therefore, in order to simplify the equipment, particularly for field applications, it is desirable to eliminate the need for changing voltage levels and/or neutron energy levels. Furthermore, the sequential impingement of two or more beams of neutrons is time consuming. Consequently, it would be desirable to have a neutron scatter based analysis system in which the need to impinge two neutron beams of different energies onto a sample in sequence is eliminated.
In accord with the present invention, it has been found that the anisotropic nature of the elastic scattering of neutrons may be exploited to simplify the aforedescribed analysis system. It has been found that the intensity of the elastically scattered neutrons exhibits a scattering angle dependence, that is to say the scattered neutrons will produce a strong signal at a first scattering angle and a significantly smaller signal at a second scattering angle. By appropriately selecting the system geometry and the neutron energy, a single neutron beam may be employed to provide both an alarm signal and a non-resonant, reference, signal. In this manner, both the apparatus and methodology for carrying out a non-invasive analysis of objects is greatly simplified. These and other advantages of the invention will be readily apparent from the drawings, discussion and description which follow: